Windshear refers to a sudden change in wind speed or direction which causes airplanes to lose airspeed and lift. Low level windshear below 1000 feet of altitude is a serious threat to airplanes that are taking off or landing. According to the Federal Aviation Administration, low level windshear has been responsible for 27 aircraft accidents between 1964 and 1985, and has caused 5 accidents in the last decade which resulted in over 500 fatalities. Consequently, windshear is considered a major hazard to aircraft and to air transportation safety.
Windshears are caused by a variety of weather conditions which are related to thermal processes in the atmosphere. These processes produce different air masses with distinct temperature differences. The most severe and unpredictable windshears are caused by a sudden downdraft of cooled air that falls from clouds in the form of a downburst of air and moisture mixed together. This phenomenon is called either a microburst or macroburst depending on its size. Macrobursts are large scale events that produce large diameter gusts of wind as a result of a severe thunderstorm or intense rainfall. In general, these type of strong gust fronts do not represent a life threatening hazard to aircraft, but they still require early recognition and proper flight management procedures in order to maintain safe flight.
A microburst is a relatively small but very intense downburst of evaporatively driven air that can reach speeds of 100 miles per hour. Microbursts range in size from under one mile to over two miles in diameter and normally last for only five or ten minutes. Because of their small scale, they are unpredictable by standard weather forecasting techniques and essentially undetectable by the human eye. When this severe downdraft approaches the ground, increased pressure forces the air out into radial, divergent winds of high velocity. Due to the small diameter of the opposing gust fronts, a plane flying through a microburst experiences a very rapid change of wind direction, or windshear. The divergence of these horizontal winds can be so great that a low flying aircraft can lose sufficient airspeed to stall and crash.
Although microbursts are being intensively researched, there is no complete understanding of how they are triggered. They appear to be related to certain disturbances that occur in the atmosphere during periods of high convective activity, similar to conditions that spawn severe thunderstorms and tornadoes. Also, hail and high altitude snow storms have been connected to the triggering of microbursts, but only when certain atmospheric conditions have already been established.
The common condition that is necessary before a microburst can occur is the development of an unstable atmosphere that approaches a condition of neutral buoyancy. When this condition occurs, particles of air that rise or fall due to thermal density differences with their surrounding air masses tend to stay in motion, rather than to be dampened out by stabilizing forces. Consequently, a large air mass such as a moisture laden cloud that is disturbed (by other atmospheric changes) can become slightly heavier than its ambient environment, and begin to accelerate downward due to gravitational forces. As the moisture laden air and precipitation mixture falls and gains speed, it begins to evaporate at a higher rate causing a temperature decrease which still further increases the density difference between it and the surrounding air. The result of this runaway, cyclic action during unstable atmospheric conditions is a high velocity downburst of air and precipitation that forms the dangerous windshears near the ground.
When the microburst hits the ground embedded in a rainfall, it is called a wet microburst. However, if the air is very dry and the microburst falls from a high altitude, most or all of the moisture within the air mass can evaporate and only the strong vertical downdraft of cool air hits the ground. This condition is called a dry microburst.
There are two general approaches to detecting hazardous windshears that occur in the path of low level aircraft. The first or so-called reactive method, is the only one in current practice aboard aircraft today. This method employs accelerometers which sense the rapidly changing winds by the aircraft's "reaction" to the winds. An associated computer produces a warning if a certain threshold of a calculated windshear hazard is exceeded. The problem with this technique is that the aircraft must first enter the dangerous windshear condition before the accelerometers can detect a changing wind. As a result, there is very little time remaining before the plane must escape the windshear to prevent a crash. This requires immediate action by the pilot to initiate an escape maneuver to allow the plane to gain airspeed and altitude as soon as possible. Due to the reaction time of both pilot and engines, there are some windshear conditions that are inescapable with this technique.
The second detection method is called "Forward Looking" and uses a remote sensor to detect the windshear before the aircraft enters it, allowing sufficient time to avoid the windshear or at least to fly through it at a safe altitude and speed. Several candidate remote sensors are Doppler Radar, Doppler Lidar, and Infrared(IR) systems. The Doppler systems rely on a transmitted beam of electromagnetic energy that is reflected off particles in the divergent air masses (i.e. rain or aerosols) which produces a Doppler shift proportional to the magnitude of the divergent winds. The infrared remote sensing technique, utilized in the present invention, relies on a passive, remote measurement of the atmospheric temperature to detect specific changes or gradients that infer the presence of hazardous windshear conditions.